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Search Results: 1 - 10 of 198113 matches for " N. Karagiannakis "
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Study of Antigravity in an Model and in Brans-Dicke Theory with Cosmological Constant
V. K. Oikonomou,N. Karagiannakis
Journal of Gravity , 2014, DOI: 10.1155/2014/625836
Abstract: We study antigravity, that is, having an effective gravitational constant with a negative sign, in scalar-tensor theories originating from theory and in a Brans-Dicke model with cosmological constant. For the theory case, we obtain the antigravity scalar-tensor theory in the Jordan frame by using a variant of the Lagrange multipliers method and we numerically study the time dependent effective gravitational constant. As we will demonstrate by using a specific model, although there is no antigravity in the initial model, it might occur or not in the scalar-tensor counterpart, mainly depending on the parameter that characterizes antigravity. Similar results hold true in the Brans-Dicke model. 1. Introduction During the last two decades our perception about the universe has changed drastically owing to the discovered late time acceleration that our universe has. Particularly, it can be thought as one of the most striking astrophysical observations with another striking observation being the verification of the inflating period of our universe. Actually, moving from time zero to present time, inflation came first, with the late time acceleration occurring at present epoch. One of the greater challenges in cosmology is to model this late time acceleration in a self-consistent way. According to the new Planck telescope observational data for the present epoch, the universe is consistently described by the model, according to which the universe is nearly spatially flat and consists of ordinary matter (~4.9%), cold dark matter (~26.8%), and dark energy (~68.3%). The dark energy is actually responsible for late time acceleration and current research on the field is mostly focused on this issue. One of the most promising and theoretically appealing descriptions of dark energy and late time acceleration issues is provided by the modified theories of gravity and related modifications. For important review articles and papers on the vast issue of theories, the reader is referred to [1–19] and references therein. For some alternative theories to modified gravity that model dark energy, see [5, 20–24]. The most appealing characteristic of modified gravity theories is that what is actually changed is not the left hand side of the Einstein equations, but the right hand side. Late time acceleration then requires a negative fluid, which can be consistently incorporated in the energy momentum tensor of these theories. This feature naturally appears in theories and also late time acceleration solutions of the Friedmann-Robertson-Walker equations naturally occur in these
Late Time Cosmological Evolution in f(R) theories with Ordinary and Collisional Matter
V. K. Oikonomou,N. Karagiannakis
Physics , 2014, DOI: 10.1088/0264-9381/32/8/085001
Abstract: We study the late time cosmological evolution of $f(R)$ theories of modified gravity, with the matter content of the universe being that of collisional self interacting matter. We assume that the universe is described by a flat Friedmann-Lemaitre-Robertson-Walker metric and that it is matter and dark energy dominated. The results of our numerical analysis for a collisional matter $f(R)$ theory are compared with those resulting from pressure-less matter $f(R)$ theory and from the $\mathrm{\Lambda}\mathrm{CDM}$ model. As we shall demonstrate, the resulting picture can vary from model to model, indicating that the effect of collisional matter in $f(R)$ theories is strongly model dependent. Particularly, in a few cases, may give better fit to the $\mathrm{\Lambda}\mathrm{CDM}$ model. In all studied cases, the effective equation of state parameter does not cross the phantom divide, both in the collisional matter and pressure-less matter $f(R)$ theories. Finally, we thoroughly study the effects of collisional matter on one of the $f(R)$ models that is known to provide a unified description of early time inflation and late time acceleration. The overall picture of the evolution of the universe is not drastically affected, apart from the matter era which is further enhanced with an additional matter energy density term, which is of leading order. However, a fully consistent description of the universe's evolution requires the introduction of a dark energy compensate in the total energy density, a concept very well known from the literature.
Cold Dark Matter and Higgs Mass in the Constrained Minimal Supersymmetric Standard Model with Generalized Yukawa Quasi-Unification
N. Karagiannakis,G. Lazarides,C. Pallis
Physics , 2013, DOI: 10.1142/S0217751X13300482
Abstract: The construction of specific supersymmetric grand unified models based on the Pati-Salam gauge group and leading to a set of Yukawa quasi-unification conditions which can allow an acceptable b-quark mass within the constrained minimal supersymmetric standard model with mu>0 is briefly reviewed. Imposing constraints from the cold dark matter abundance in the universe, B physics, and the mass mh of the lighter neutral CP-even Higgs boson, we find that there is an allowed parameter space with, approximately, 44
Dark Matter and Higgs Mass in the CMSSM with Yukawa Quasi-Unification
N. Karagiannakis,G. Lazarides,C. Pallis
Physics , 2012, DOI: 10.1088/1742-6596/384/1/012012
Abstract: We present an updated analysis of the constrained minimal supersymmetric standard model with mu>0 supplemented by an `asymptotic' Yukawa coupling quasi-unification condition, which allows an acceptable b-quark mass. Imposing constraints from the cold dark matter abundance in the universe, B physics, the muon anomalous magnetic moment, and the mass m_h of the lightest neutral CP-even Higgs boson, we find that the lightest neutralino cannot act as a cold dark matter candidate. This is mainly because the upper bound on the lightest neutralino relic abundance from cold dark matter considerations, despite the fact that this abundance is drastically reduced by neutralino-stau coannihilations, is incompatible with the recent data on the branching ratio of B_s --> mu^+ mu^-. Allowing for a different particle, such as the axino or the gravitino, to be the lightest supersymmetric particle and, thus, constitute the cold dark matter in the universe, we find that the predicted m_h's in our model favor the range (119-126) GeV.
Is Post-ERCP Pancreatitis a Genetically Predisposed Complication?
Konstantinos Mystakidis,George Kouklakis,Androniki Papoutsi,Vasilios D. Souftas,Eleni Efremidou,Dimitrios Kapetanos,Michail Pitiakoudis,Nikolaos Lyratzopoulos,Anastasios Karagiannakis,Alexandros Pantelios
Gastroenterology Research and Practice , 2012, DOI: 10.1155/2012/473960
Abstract: Background/Objectives. Pancreatitis remains the most common complication of ERCP. History of post-ERCP pancreatitis is an independent risk factor for a new episode, suggesting a genetic background. The N34S mutation in serine protease inhibitor Kazal type 1 (SPINK 1) gene may downregulate the threshold for the development of pancreatitis. The aim of the present study is to evaluate the presence of this mutation among patients with post-ERCP pancreatitis. Methods. During a period of four years, thirty patients with post-ERCP pancreatitis entered the study. Patients and procedural data were collected, focusing on risk factors for pancreatitis. Blood samples were taken for genetic testing for the presence of N34S mutation in SPINK 1 gene. After DNA extraction, we used an allele-specific polymerase chain reaction as an initial screening method for the N34S mutation, and in order to confirm the results and to determine the hetero- and homozygosity genotype status, we used a restriction fragment length polymorphism (RFLP) method. Results. None of the thirty patients was found to carry the N34S mutation, with both of the applied methods. Patients had an average of two of the known risk factors. Conclusion. SPINK1 N34S mutation does not seem to play a role in post-ERCP pancreatitis, but larger studies needed to confirm our results. 1. Introduction Pancreatitis remains the most common complication of ERCP, with the reported incidence ranging from 2% to 9% [1]. Although 80% of cases are mild, a significant number of patients may develop severe pancreatitis, that means additional morbidity and risk for death. ERCP, despite the development of new diagnostic tools, remains a widely used procedure, so post-ERCP pancreatitis is a problem with significant impact. So far only the use of pancreatic stents in high risk patients has become a widely accepted practice to minimize the risk for post-ERCP pancreatitis [2]. However, this technique is costly and not widely available, so the question who are high risk patients remains vivid. Several studies and meta-analyses helped us to recognize special factors that put an individual in high risk for the development of post-ERCP pancreatitis [3–10]. Among these factors special interest presents the history of post-ERCP pancreatitis as an independent risk factor for a new episode of post-ERCP pancreatitis. It seems that some individuals have a genetically predisposed susceptibility in this particular complication. Genetic factors, such mutations in genes of cationic trypsinogen and CFTR, are known to play a causal role in the
Implementation of a Higher Quality dc Power Converter  [PDF]
N. N. Barsoum
Journal of Electromagnetic Analysis and Applications (JEMAA) , 2010, DOI: 10.4236/jemaa.2010.22012
Abstract: Many single and three-phase converters are well developed, and covered up in most of electric markets. It is used in many applications in power systems and machine drives. However, an exact definite output signal from the dc side still not recognized. The waveforms of output voltage and current demonstrate an imperfect dc signal and constitute losses, harmonic distortion, low power factor, and observed some ripples. An approximately perfect rectifier bridge is the aim of this research. Perhaps it gives the ability to identify the parameters of the converter to obtain, as much as possible, a perfect dc signal with less ripple, high power factor and high efficiency. Design is implemented by simulation on Power Simulator PSIM, and practically, a series regulator LM723 is applied to provide regulating output voltage. Comparisons of both simulation and hardware results are made to observe differences and similarities.
Estimation of Sensitivity of the DS/AHP Method While Solving Foresight Problems with Incomplete Data  [PDF]
N. Pankratova, N. Nedashkovskaya
Intelligent Control and Automation (ICA) , 2013, DOI: 10.4236/ica.2013.41011
Abstract:

The paper provides mathematical analysis of sensitivity of different combination rules in the DS/AHP method when an alternative is added to the set of decision alternatives while solving foresight problems. Different cases of rank reversals are defined and two sets of conditions for these cases using the method DS/AHP are considered. Rank reversals are illustrated when the DS/AHP method is used to solve practical problem of critical technologies of energy conservation and power efficiency evaluation in Ukraine. It is shown that the DS/AHP method is not sensitive to exclusion (or addition) of an irrelevant decision alternative from (or to) the set of decision alternatives.

Computation of the Genetic Code: Full Version  [PDF]
N. N. Kozlov
Journal of Computer and Communications (JCC) , 2017, DOI: 10.4236/jcc.2017.510008
Abstract: One of the problems in the development of mathematical theory of the genetic code (summary is presented in [1], the detailed—to [2]) is the problem of the calculation of the genetic code. Similar problem in the world is unknown and could be delivered only in the 21st century. One approach to solving this problem is devoted to this work. For the first time a detailed description of the method of calculation of the genetic code was provided, the idea of which was first published earlier [3]), and the choice of one of the most important sets for the calculation was based on an article [4]. Such a set of amino acid corresponds to a complete set of representation of the plurality of overlapping triple gene belonging to the same DNA strand. A separate issue was the initial point, triggering an iterative search process all codes submitted by the initial data. Mathematical analysis has shown that the said set contains some ambiguities, which have been founded because of our proposed compressed representation of the set. As a result, the developed method of calculation was reduced to two main stages of research, where at the first stage only single-valued domains were used in the calculations. The proposed approach made it possible to significantly reduce the amount of computation at each step in this complex discrete structure.
The Study of the Secrets of the Genetic Code  [PDF]
N. N Kozlov
Journal of Computer and Communications (JCC) , 2018, DOI: 10.4236/jcc.2018.67007
Abstract: The disclosure of many secrets of the genetic code was facilitated by the fact that it was carried out on the basis of mathematical analysis of experimental data: the diversity of genes, their structures and genetic codes. New properties of the genetic code are presented and its most important integral characteristics are established. Two groups of such characteristics were distinguished. The first group refers to the integral characteristics for the areas of DNA, where genes are broken down in pairs and all 5 cases of overlap, allowed by the structure of DNA, were investigated. The second group of characteristics refers to the most extended areas of DNA in which there is no genetic overlap. The interrelation of the established integral characteristics in these groups is shown. As a result, a number of previously unknown effects were discovered. It was possible to establish two functions in which all the over-understood codons in mitochondrial genetic codes (human and other organizations) participate, as well as a significant difference in the integral characteristics of such codes compared to the standard code. Other properties of the structure of the genetic code following from the obtained results are also established. The obtained results allowed us to set and solve one of the new breakthrough problems—the calculation of the genetic code. The full version of the solution to this problem was published in this journal in August 2017.
Guidelines for Optimization of the Absorber Layer Energy Gap for High Efficiency Cu(In,Ga)Se2 Solar Cells  [PDF]
N. Severino, N. Bednar, N. Adamovic
Journal of Materials Science and Chemical Engineering (MSCE) , 2018, DOI: 10.4236/msce.2018.64015
Abstract: This work investigates in-depth the effects of variation of the compositional ratio of the absorber layer in Cu(In,Ga)Se2 (CIGS) thin-film solar cells. Electrical simulations were carried out in order to propose the most suitable gallium double-grading profile for the high efficiency devices. To keep the model as close as possible to the real behavior of the thin film solar cell a trap model was implemented to describe the bulk defects in the absorber layer. The performance of a solar cell with a standard CIGS layer thickness (2 μm) exhibits a strong dependence on the front grading height (decreasing band gap toward the middle of the CIGS layer). An absolute gain in the efficiency (higher than 1%) is observed by a front grading height of 0.22. Moreover, simulation results show that the position of the plateau (the region characterized by the minimum band gap) should be accurately positioned at a compositional ratio of 20% Ga and 80% In, which corresponds to the region where a lower bulk defect density is expected. The developed model demonstrates that the length of the plateau is not playing a relevant role, causing just a slight change in the solar cell performances. Devices with different absorber layer thicknesses were simulated. The highest efficiency is obtained for a CIGS thin film with thicknesses between 0.8 and 1.1 μm.
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